The present invention relates to a process for producing semiconductor devices. More specifically, the invention relates to a process for producing semiconductor devices in which an intrinsic base region of a bipolar transistor is reliably connected to a graft base region thereof.
Referring to the base of a bipolar transistor as is well known, a portion formed just under the emitter region is called intrinsic base region, and a portion outside the intrinsic base portion, i.e., a portion where a base contact is formed, is called graft base region or external base region.
So far, bipolar transistors have generally been manufactured by a process according to which the graft base region is formed, and the intrinsic base region is formed that portions of the two regions are overlapped.
In recent years, however, there has been proposed a method according to which the graft base region and the emitter region are formed by the self-alignment technique to increase the degree of integration of semiconductor devices and improving characteristics of the semiconductor devices at high frequencies (see, for example, Japanese Patent Publication No. 27469/1980).
An example of such a method will be explained below in conjunction with FIGS. 1a and 1b.
First, as shown in FIG. 1a, in a case of an npn-type transistor will be formed a thick silicon dioxide film 2' is selectively formed by the well-known LOCOS method, on the surface of the silicon substrate 1' to isolate elements from each other. In this case, a polycrystalline silicon film 3' heavily doped with boron ions is formed and then, the unnecessary portion is formed to form an opening 6'.
Next, a silicon dioxide film 4' is formed on the surface of the polycrystalline silicon film 3' by thermal oxidation, by utilizing the phenomenon that polycrystalline silicon heavily doped with boron ions undergoes oxidation at a very high rate. At this moment, boron ions contained in the polycrystalline silicon film 3' are diffused into the silicon substrate 1', so that a graft base region 5' is formed simultaneously with the silicon dioxide film 4'.
Based upon the ion implantation technique, impurities for forming the base region are, first, implanted into the surface region of the silicon substrate 1 as indicated by arrows 10' and then impurities for forming the emitter region are implanted thereinto, followed by the heating. Thus, there are formed an intrinsic base region 7' and an emitter region 8' as shown in FIG. 1b.
When a gap .DELTA.BE is small between the graft base region 5' and the emitter region 8', the graft base region 5' and the intrinsic base region 7' are connected to each other making it possible to form a good bipolar transistor.
When the gap .DELTA.BE is large, however, the graft base region 5' and the intrinsic base region 7' are formed being separated from each other as shown in FIG. 2, and the base resistance increases remarkably.
Further, depths of the base region and emitter region decrease remarkably with the decrease in the size of the transistor. To connect such a very shallow intrinsic base region to the graft base region without interruption, however, the gap .DELTA.BE must be controlled maintaining very high precision.
It is, however, very difficult to control the gap between the graft base region and the emitter region maintaining high precision, and it is further difficult to prevent the graft base region from separating from the intrinsic base region. These facts account for variance in the characteristics of bipolar transistors and for the production of unacceptable products.